1. Field of the Invention
The present invention relates to a circuit for detecting a light-received position in a light receiving portion such as, for example, a light-received position detecting circuit to be used in a distance detecting apparatus for automatically focusing in a camera.
2. Description of the Related Art
In a distance detecting apparatus used for an automatically focusing function in a camera or the like, conventionally, a position sensitive light detector (PSD) is used a discrete part.
FIG. 7 shows the schematic configuration of a part of a distance detecting apparatus using such a PSD 50. The PSD 50 is a semiconductor element for converting received light into a photoelectric current part by using a pn junction formed in a semiconductor substrate. A pair of electrodes (not shown) each for taking out a photoelectric current part are formed on the opposite ends of the PSD 50. An internal resistance having a predetermined resistance value formed by a p- or n-layer of the semiconductor is present between the electrodes so as to divide a photoelectric current generated in the PSD 50 into photoelectric current parts according to the position where the photoelectric current is generated. The photoelectric current parts are taken out from the electrodes on the opposite ends, respectively. The photoelectric current parts taken out from the respective electrodes are inversely proportional to the distances from the position of generation of the photoelectric current to the electrodes respectively. Amplifiers 52 and 54 with their input terminals connected to the respective electrodes compare the photoelectric current parts taken out from the respective electrodes with a reference voltage V.sub.ref to thereby generate predetermined detection signals respectively. Output terminals of the amplifiers 52 and 54 are connected to two input terminals of an amplifier 56 so that the amplifier 56 compares the output signals of the respective amplifiers 52 and 54 to thereby generate a predetermined detection signal. A not-shown arithmetic operation portion calculates the light-received position on the PSD 50 on the basis of the detection signal generated by the amplifier 56.
For example, in a distance detecting apparatus in a camera, or the like, light is radiated from a not-shown light emitting portion to a target such as a subject or the like, the position of the PSD 50 where the light reflected from the target enters is detected, and the distance from the camera to the target is calculated on the basis of the detected light-received position. Further, the lens position of the camera is moved in accordance with the detected distance to thereby perform the focusing of the camera.
In the PSD 50, however, there was such a problem that the photoelectric current part generated by light reception was so small (for example, an order of n A) that the photoelectric current part was easily affected by external noise. Especially, because the PSD 50 was configured as a discrete part, there was a high possibility that external noise was superimposed on signals in the wiring paths from the opposite ends of the PSD 50 to the amplifiers 52 and 54 respectively. In order to reduce such an influence of noise, it was thought of a method of increasing the gain of the amplifiers 52 and 54. In such a method, however, the noise level became high as the gain increased so that it was impossible to improve the S-N ratio.
Further, it was thought of a method in which the PSD 50 and the amplifiers 52 and 54 were provided, for example, as an integrated circuit to make the wiring paths short. In this case, however, the production of the internal resistance of the PSD 50 was under restrictions (particularly, impurity concentration) in process of producing semiconductor devices such as transistors, etc. so that an internal resistance having a desired resistance value necessary for detecting a very small photoelectric current part could not be easily formed. Accordingly, this method was not practical.
In a light-received position detecting circuit using such a PSD 50, conventionally, the wiring paths for the PSD 50 to the respective amplifiers 52 and 54 were made as short as possible and the wirings were shielded so that the effects of the noise was reduced to keep necessary accuracy in detecting the light-received position. There was therefore such a problem that the degree of freedom was limited in apparatus design, the assembling cost was high, and so on.
Further, in order to improve the accuracy in distance detection, the light-received position detecting operation was repeated many times, and the obtained values of the light-received position or the detected values of the distance to the target were averaged. Accordingly, there was such a problem that it took a long time for distance detection.